The present invention relates to a piston seal member and a disc brake using the piston seal member.
A piston seal member is generally formed of a crosslinked rubber composition containing rubber as the main component. For example, a vehicular disc brake is equipped with a caliper body including a piston and a cylinder. A piston seal member is provided in a ring-shaped groove formed in the inner circumferential surface of the cylinder. In the disc brake, brake pads are pressed against a disc rotor secured on each wheel by applying hydraulic brake pressure to stop rotation of the wheel due to the friction force of the brake pads made of a friction material. The piston seal member has a role of sealing a brake fluid and a role of returning (rolling back) the piston which has moved forward due to hydraulic brake pressure. The brake pads are pressed against the disc by causing the piston to move forward in a cylinder bore by applying hydraulic brake pressure.
Specifically, the piston seal member enables the cylinder and the piston inserted into the cylinder bore to come into contact in a fluid-tight and movable manner. The piston which has moved forward due to hydraulic pressure is rolled back by the piston seal member (see JP-B-3-59291, for example). Therefore, the piston seal member is required to have toughness for securely sealing the brake fluid and elasticity for returning (rolling back) the piston which has moved forward due to hydraulic pressure.
The temperature of the caliper body of the disc brake increases during the operation due to frictional heat which occurs between the disc rotor and the brake pads. Therefore, the piston seal member is also subjected to high temperature. The piston seal member formed of a rubber composition thermally expands and exhibits a decreased modulus of elasticity at a high temperature. In this case, the amount of rollback of the piston changes due to thermal expansion and a decrease in modulus of elasticity of the piston seal member, whereby the working allowance of the brake changes. For example, the amount of stroke of a brake lever for a motorcycle disc brake changes, whereby the driver may feel that the brake operation is incorrect.
To deal with this problem, a piston seal member has been proposed which is formed of a rubber composition in which at least 100 parts by weight of carbon black is added based on 100 parts by weight of ethylene-propylene rubber (see JP-A-2004-316773, for example).
A piston seal member has also been proposed which is formed of a rubber containing at least either carbon fibers (including carbon nanofibers) or fullerene (see JP-A-2004-232786, for example).
However, a piston seal member exhibiting durability to a brake fluid at a high temperature of 200° C. or more has not yet been developed.